1. Field of the Invention
The present invention relates to a motor driving system configured to drive a motor such as a fan motor that forcibly circulates cold air in a freezing showcase or a refrigerator.
2. Description of the Related Art
In a fan motor for forcibly circulating cold air in a freezing showcase or a refrigerator, it is conventionally required to keep a rotation speed constant, to change over a rotation speed depending on the in-refrigerator situation, or to change a rotation speed pursuant to a predetermined program. The control of a rotation speed is performed by a motor driving system. From the viewpoint of electric power saving and ease of control, a brushless motor is often used as a motor to be driven by the motor driving system of this kind. In this case, some components of the motor driving system such as a rectifying circuit and a drive circuit for supplying an electric current to the brushless motor are arranged within a motor case.
In recent years, it is required for a convenience store or a supermarket to intensively or individually manage a plurality of showcases. This makes it necessary to individually control the rotation speeds of fan motors provided in the respective showcases. For that reason, the motor driving system is provided with an interface through which the rotation speeds can be controlled from the outside.
FIG. 6 is a block diagram showing a conventional motor driving system including an interface through which the rotation speeds can be controlled from the outside and a driving target. The driving target of the motor driving system 1 is a fan motor 2 which is a three-phase brushless motor. In the motor driving system 1, from a commercial power supply 3, AC 100 V or AC 200 V is inputted via a noise filter 4 and a reactor 5 and is rectified by a rectifying circuit 6, thereby generating a DC voltage of 140 V or 280 V. Reference symbol 7 designates a smoothing capacitor. In the motor driving system 1, a regulator 8 as a switching power supply generates an operating voltage from the DC voltage generated in the rectifying circuit 6.
In the motor driving system 1, a speed command 9 as a pulse width modulation signal is inputted from an input device 10 to a drive circuit 11. The drive circuit 11 receives output signals of hall elements 10A, 10B and 10C provided in the fan motor 2. The drive circuit 11 generates and outputs a drive signal so that the present rotation speed determined by the output signals of the hall elements 10A, 10B and 10C can become equal to the rotation speed indicated by the speed command. Responsive to the drive signal outputted from the drive circuit 11, a power module 12 outputs the DC voltage generated from the rectifying circuit 6 to the respective phases U, V, and W of the fan motor 2. Consequently, the motor driving system 1 rotationally drives the fan motor 2 at the rotation speed indicated by the speed command. In the motor driving system 1, the driving current of the fan motor 2 driven in this manner is detected by the drive circuit 11 using a terminal voltage of a current detecting element 13 formed of a resistor. The motor driving system 1 controls the drive timing of the power module 12 pursuant to the output signals of the hall elements 10A, 10B and 10C such that the driving current does not exceed a specified reference value. In the motor driving system 1 shown in FIG. 6, the rectifying circuit 6, the smoothing capacitor 7, the regulator 8, the hall elements 10A, 10B and 10C, the drive circuit 11, the power module 12 and the current detecting element 13 are arranged within a motor case.
In a case where an interface through which the rotation speeds can be controlled from the outside of the showcases is provided, it is necessary for the input device 10 associated with the interface to comply with the same safety standards as the safety standards required in the fan motor. More specifically, it is required, for a fan motor driving system connected to a commercial power supply, to comply with the safety standards such as UL (Underwriters Laboratories) and the like which are applied to the commercial power supply. Unless a measure is taken otherwise, the input device 10 associated with the speed control needs to satisfy the safety standards. Large-scale measures need to be taken against electric shocks, lightning surges, noises and so forth. As a result, there is a problem in that the configuration of the motor driving system becomes complex and troublesome.